Manufacturing method of porous silicon material, porous silicon material, and power storage device

ABSTRACT

The manufacturing method of a porous silicon material of the present disclosure includes a particle forming step of melting a raw material containing Al as a first element in an amount of 50% by mass or more and Si in an amount of 50% by mass or less to obtain a silicon alloy, a pore forming step of removing the first element from the silicon alloy to obtain a porous material, and a heat treatment step of heating the porous material to diffuse elements other than Si to a surface of the porous material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. application Ser.No. 17/844,175, filed on Jun. 20, 2022, which claims priority toJapanese Patent Application No. 2021-102903 filed on Jun. 22, 2021, theentire disclosures of which are incorporated herein by reference.

BACKGROUND 1. Technical Field

The present specification discloses a manufacturing method of a poroussilicon material, a porous silicon material, and a power storage device.

2. Description of Related Art

Conventionally, as a negative electrode material of silicon, poroussilicon has been suggested which is obtained by mixing 70 parts by massof Si lumps with 30 parts by mass of Al powder, then making the mixtureinto a molten alloy in an argon atmosphere, making the alloy intoparticles by a gas atomization method using a helium gas, and thenremoving Al by using hydrochloric acid (for example, see JapaneseUnexamined Patent Application Publication No. 2004-214054 (JP2004-214054 A)). According to JP 2004-214054 A, the porous silicon makesit possible to completely suppress micronization resulting from thevolumetric expansion and contraction of an active material during chargeand discharge, peeling of the active material from a current collector,and lack of contact with a conductive material. As a manufacturingmethod of a silicon material, a method has been suggested which includesseparating a Si alloy composed of Si and an intermediate alloy elementincluding Mg, Co, Cr, Cu, Fe, or the like into Si fine particles and asecond phase formed by the substitution of the intermediate alloyelement with a molten element in a molten metal containing apredetermined molten element and removing the second phase to obtain aporous silicon material (for example, see Japanese Unexamined PatentApplication Publication No. 2012-82125 (JP 2012-82125 A)). According toJP 2012-82125 A, with the porous silicon material, a high capacity andhigh cycle characteristics can be obtained.

SUMMARY

However, using an alloy having a Si content of 50% by mass or more, themanufacturing method in JP 2004-214054 A is not effective enough toreduce the problem resulting from the expansion and contraction of Si.In the manufacturing method of a porous silicon material in JP2012-82125 A, the silicon alloy containing the intermediate alloyelement needs to be melted and substituted in a molten metal containinga predetermined molten element, that is, the silicon alloy needs to betreated at a high temperature, which necessitates a simple manufacturingprocess.

The present disclosure has been made in consideration of the aboveproblems, and the main object of the present disclosure is to provide amanufacturing method of a porous silicon material that can furtherimprove electrochemical characteristics, a porous silicon material, anda power storage device.

As a result of carrying out intensive studies to achieve theaforementioned object, the inventors of the present disclosure havefound that preparing a silicon alloy containing Al at a mixing ratioclose to that in a eutectic composition, removing Al, and performing aheating treatment enables Al present as a solid solution in silicon todiffuse to the surface of the alloy, which makes it possible to obtain aporous silicon material that can further improve electrochemicalcharacteristics. Based on the finding, the inventors have accomplishedthe manufacturing method of a porous silicon material, the poroussilicon material, and the power storage device of the presentdisclosure.

That is, the manufacturing method of a porous silicon material of thepresent disclosure includes a particle forming step of making a siliconalloy that contains Al as a first element in an amount of 50% by mass ormore and Si in an amount of 50% by mass or less into particles, a poreforming step of removing the first element from the silicon alloy toobtain a porous material, and a heat treatment step of heating theporous material to diffuse elements other than Si to a surface of theporous material.

The porous silicon material of the present disclosure contains skeletalsilicon with a three-dimensional network structure having voids that isformed of silicon having a lattice constant of 5.435 Å or less, in whichan average porosity of the porous silicon material is in a range of 50%by volume or more and 95% by volume or less, a proportion of Sicontained in the porous silicon material as an element excluding oxygenis 85% by mass or more, a proportion of Al contained in the poroussilicon material as a first element is in a range of 15% by mass orless, and Al is present on a surface of the porous silicon material.

The power storage device of the present disclosure includes a positiveelectrode that contains a positive electrode active material, a negativeelectrode that contains the aforementioned porous silicon material as anegative electrode active material, and an ion conducting medium that isinterposed between the positive electrode and the negative electrode andconducts carrier ions.

The present disclosure can further improve the electrochemicalcharacteristics of materials containing Si. The reason why such aneffect is obtained is presumed as follows. For example, in a lithium ionsecondary battery, a silicon electrode has a theoretical capacity of4,199 mAh/g, which is about 10 times higher than theoretical capacity,372 mAh/g, of general graphite. Therefore, the silicon electrode isexpected to further increase capacity and energy density. Meanwhile,silicon having absorbed lithium ions is Li_(4.4)Si, which has a volumehaving expanded about 4 times the volume of silicon not yet absorbinglithium. According to the present disclosure, a substance other thansilicon that is mainly Al and incorporated into a silicon alloy isselectively removed by being dissolved, which makes it possible toeasily produce a porous silicon material having a small pore size and ahigh porosity. When the porous silicon material having a small pore sizeand a high porosity is used in a power storage device, such as a lithiumion secondary battery, the expansion and contraction of volume aresignificantly mitigated. Accordingly, for example, charge and dischargecharacteristics, such as charge and discharge cycle characteristics, canbe improved, which makes it possible to easily obtain a high-performancepower storage device. In the porous silicon material, other elements,such as Al, are diffused on the surface. Therefore, the porous siliconmaterial has a purer silicon skeleton, which can further improveelectrochemical characteristics, such as charge and dischargecapacities.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like signs denote likeelements, and wherein:

FIG. 1 is an Al—Si binary phase diagram;

FIG. 2 is a view illustrating an example of the structure of a powerstorage device 10;

FIG. 3 is an SEM image of an Al—Si alloy observed after gas atomizationin Experimental Example 1;

FIG. 4A is an SEM image of a porous silicon material of ExperimentalExample 1;

FIG. 4B is an SEM image of the porous silicon material of ExperimentalExample 1;

FIG. 4C is an SEM image of the porous silicon material of ExperimentalExample 1;

FIG. 4D is an EDAX mapping image of the porous silicon material ofExperimental Example 1;

FIG. 5 is a pore distribution curve of the porous silicon material ofExperimental Example 1;

FIG. 6 is an SEM image of the porous silicon material of ExperimentalExample 8;

FIG. 7 is a pore distribution curve of the porous silicon material ofExperimental Example 8;

FIG. 8 is a diagram showing the relationship between heat treatmenttemperatures and XRD profiles in Experimental Example 8;

FIG. 9 is a diagram showing the relationship between heat treatmenttemperatures and a lattice constant a_(Si) in Experimental Example 8;

FIG. 10 is a diagram showing the relationship between heat treatmenttemperatures and amounts of Al dissolved (% by mass) in ExperimentalExample 8;

FIG. 11A is a view illustrating how the state of a porous siliconmaterial changes by a treatment;

FIG. 11B is a view illustrating how the state of a porous siliconmaterial changes by a treatment;

FIG. 11C is a view illustrating how the state of a porous siliconmaterial changes by a treatment; and

FIG. 11D is a view illustrating how the state of a porous siliconmaterial changes by a treatment.

DETAILED DESCRIPTION Manufacturing Method of Porous Silicon Material

The manufacturing method of a porous silicon material of the presentdisclosure includes a particle forming step, a pore forming step, and aheat treatment step. In the particle forming step, a treatment of makinga silicon alloy containing Al as a first element and Si into particlesis performed. In the pore forming step, a treatment of removing thefirst element from the silicon alloy to obtain a porous material isperformed. In the heat treatment step, a treatment of heating the porousmaterial to diffuse elements other than Si to a surface of the porousmaterial is performed.

Particle Forming Step

In the particle forming step, a silicon alloy is used in which thecontent of the first element is in a range of 50% by mass or more andthe content of Si is in a range of 50% by mass or less. Examples of thefirst element include Al. Examples of raw materials used in the particleforming step include a Si metal and an Al metal. The content of thefirst element in the silicon alloy used in the particle forming step is,in embodiments, in a range of 60% by mass or more and, in someembodiments, in a range of 70% by mass or more, or may be in a range of80% by mass or more. The content of the first element in the siliconalloy used in the particle forming step is, in embodiments, in a rangeof 92% by mass or less and, in some embodiments, in a range of 90% bymass or less, or may be in a range of 85% by mass or less. The siliconalloy in which the content of the first element is in the range may beused in embodiments because the silicon alloy can further increaseporosity and makes it possible to obtain voids having more suitableshape and size. In a case where the content of the first element is 92%by mass or less, the silicon skeleton can be maintained. In a case wherethe content of the first element is 50% by mass or more, the porositycan be further increased. In a case where the content of Al is high, asingle phase of Al is precipitated much after the silicon alloy is madeinto an alloy by melting and then rapidly cooled. Accordingly, manyvoids can be formed. In the particle forming step, a silicon alloy inwhich the content of the first element is in a range capable of forminga eutectic composition may be used. The eutectic composition contains87.6% by mass of Al and 12.6% by mass of Si. However, the silicon alloymay be close to the eutectic composition, or may cover a predeterminedrange, such as a part of a hypoeutectic or hypereutectic composition.For example, the silicon alloy may be in a range of ±3% by mass withrespect to the eutectic composition. FIG. 1 is an equilibrium phasediagram of an Al—Si binary system.

In the particle forming step, the silicon alloy may be a compositionthat contains, in addition to the first element and Si, a second elementincluding one or more elements among Ca, Cu, Mg, Na, Sr, and P. Amongthese, one or more elements among Ca, Na, and Sr may be the secondelement, in embodiments. The content of the second element is lower thanthe content of the first element. For example, in the silicon alloy, thecontent of the second element with respect to the total mass of thesilicon alloy is, in embodiments, in a range of 10% by mass or less,and, in embodiments, in a range of 5% by mass or less. The content ofthe second element may be 0.1% by mass or more.

In the particle forming step, the raw materials may contain unavoidableimpurities. The unavoidable impurities are components that inevitablyremain in the process of purifying any of Si, Ti, and Al. Examples ofthe unavoidable impurities include Fe, C, Cu, Ni, and P. In embodiments,the content of the unavoidable impurities may be lower. For example, onthe assumption that the total amount of Si and Al is 100 at %, thecontent of the unavoidable impurities is 5 at % or less, in embodiments,and 2 at % or less, in some embodiments. The raw materials used in theparticle forming step may be composed of Si in a predetermined range andAl plus unavoidable impurities as a remainder. The content of Si in apredetermined range may, for example, be in a range of 10 at % or moreand 28 at % or less.

The particle forming step may be a step of making the molten siliconalloy into particles by any of a gas atomization method, a wateratomization method, and a roll quenching method. The particle formingstep may also be a step of casting the molten raw material of thesilicon alloy into a mold and crushing the obtained ingot to formparticles. Among the above methods, the gas atomization method may beused in embodiments as a method for making the silicon alloy intoparticles. During gas atomization, the alloy may be melted in an Aratmosphere, and particles may be formed in an Ar or He atmosphere. Inthe particle forming step, the silicon alloy may be made into particleshaving an average particle size in a range of 0.1 μm or more and 100 μmor less. The average particle size of the particles is, for example, 0.5μm or more and 10 μm or less, in embodiments, 1 μm or more and 5 μm orless, and, in embodiments, 1 μm or more and 3 μm or less. The siliconalloy particles may be appropriately selected depending on thecharacteristics that the power storage device needs to have. The averageparticle size of the particles is determined by observing the particleswith a scanning electron microscope (SEM), adding up the major diametersof the particles as diameters of the particles, and dividing the resultby the number of particles, so that the average particle size isobtained.

Pore Forming Step

In the pore forming step, a treatment of removing substances other thanSi from the silicon alloy prepared in the particle forming step isperformed. Examples of the substances other than Si include Al as thefirst element or an Al compound and the second element or a compound ofthe second element. In the pore forming step, an acid or alkali may beused to selectively remove Al as the first element or an Al compound orremove the second element or a compound of the second element. The acidor alkali to be used may be a substance that elutes elements and/orcompounds other than silicon in the silicon alloy but does not elutesilicon. Examples of the acid or alkali include hydrochloric acid,sulfuric acid, and sodium hydroxide. The acid or alkali may be anaqueous solution, in embodiments. The concentration of the acid oralkali is not particularly limited as long as Al as the first element oran Al compound and the second element or a compound of the secondelement can be removed. For example, the concentration of the acid oralkali can be in a range of 1 mol/L or more and 5 mol/L or less. Theremoval treatment may be performed by heating at, for example, 20° C. to60° C. In the removal treatment, the silicon alloy particles may beimmersed in an acid or alkali solution and the solution may be stirredfor about 1 hour to 5 hours. The obtained porous silicon material isthen washed and dried.

In the pore forming step, the amount of substances other than Si to beremoved may be in a range of 85% by mass or more and 100% by mass orless. For example, it does not matter even though the first or secondelement and oxygen other than the first and second elements may remain.However, because these are components unnecessary for the porous siliconmaterial, the amount of these be smaller, in embodiments. The poreforming step may also be a step of obtaining a porous silicon materialhaving an average porosity in a range of 50% by volume or more and 95%by volume or less. The average porosity is a value measured by a mercuryporosimeter.

Heat Treatment Step

In the heat treatment step, a treatment of heating the porous materialobtained by the pore forming step to diffuse elements other than Si tothe surface of the porous material is performed. In the heat treatmentstep, the porous material may be heated at a temperature in a range of400° C. or higher and 1,100° C. or lower. In this temperature range, ata temperature of 400° C. or higher, Al that is present as a solidsolution in the silicon skeleton is precipitated. Presumably, at atemperature of 600° C. or higher, the precipitated Al may diffuse on thesurface. In addition, presumably, at a temperature of 800° C. or higher,Al having diffused on the surface of the silicon skeleton may form anoxide and strengthen the silicon skeleton. In embodiments, in the heattreatment step, the first element having diffused on the surface may beoxidized and the surface of the silicon skeleton may be coated with theoxide containing the first element. Coating with the oxide makes itpossible to further strengthen the silicon skeleton. The oxide mayinclude an AlSi oxide, an AlFe oxide, a SiFe oxide, an AlSiFe oxide, andthe like, in addition to an Al oxide.

In the heat treatment step, the porous material may be heated in atemperature range of 600° C. or higher in an inert atmosphere. In a casewhere the porous material is heated under this condition, Al isprecipitated and diffuses on the surface. At the time, the porousmaterial may be heated in a temperature range of 800° C. or higher in aninert atmosphere. In a case where the porous material is heated underthis condition, the surface of the silicon skeleton can be coated withan oxide containing Al. Examples of the inert atmosphere include anitrogen atmosphere and a noble gas atmosphere. The inert atmosphere isa noble gas atmosphere, in embodiments. Examples of the noble gasinclude He and Ar. In embodiments, the noble gas may comprise Ar.Alternatively, in the heat treatment step, the porous material may beheated in the presence of oxygen at a temperature in a range of 400° C.or higher. In a case where the porous material is heated under thiscondition, Al can be precipitated, and an oxide can be produced. “In thepresence of oxygen” means mild conditions that do not induceoveroxidation of the silicon skeleton. For example, the oxygen contentin such conditions may be in a range of 5% by volume or less or 1% byvolume or less, or may be 1,000 ppm or less or 100 ppm or less. The heattreatment in the presence of oxygen is, in embodiments, performed at alower temperature compared to the heat treatment in the inertatmosphere. The temperature is, in embodiments 900° C. or lower and, inembodiments, 700° C. or lower, or may be 600° C. or lower.

Porous Silicon Material

The porous silicon material of the present disclosure is prepared by theaforementioned manufacturing method. The porous silicon materialcontains skeletal silicon (also called a silicon skeleton) having athree-dimensional network structure having voids that is formed ofsilicon having a lattice constant of 5.435 Å or less. The averageporosity of the porous silicon material is in a range of 50% by volumeor more and 95% by volume or less. A proportion of Si contained in theporous silicon material as an element excluding oxygen is 85% by mass ormore, a proportion of Al contained in the porous silicon material as afirst element is in a range of 15% by mass or less, and Al is present ona surface of the porous silicon material. The skeletal silicon has alattice constant of 5.435 Å or less, and can be regarded as having nosolid solution of Al. In embodiments, the porous silicon material mayhave a higher average porosity which is, in embodiments, 60% by volumeor more, in embodiments, 70% by volume or more, and, in embodiments, 80%by volume or more. Because the silicon skeleton needs to be in theporous silicon material, the average porosity is, in embodiments, 95% byvolume or less, in embodiments, 90% by volume or less, and, inembodiments, 86% by volume or less. In the porous silicon material, thepore size of the voids is, in embodiments, in a range of 1 nm or moreand 1 μm or less, or may be 10 nm or more, 50 nm or more, or 100 nm ormore. The pore size of the voids may be 500 nm or less, 300 nm or less,or 250 nm or less.

The porous silicon material is, in embodiments, in the form of particleshaving an average particle size of 0.1 μm or more. The average particlesize is, in embodiments, 1 μm or more, or may be 5 μm or more. Theaverage particle size of the particles is, in embodiments, 10 μm orless, in embodiments, 5 μm or less, and, in embodiments, 3 μm or less.The proportion of Si contained in the porous silicon material as anelement excluding oxygen is 85% by mass or more. In embodiments, theproportion of Si contained in the porous silicon material may be higher.The proportion of Si may be 90% by mass or more, 94% by mass or more, or96% by mass or more. The Si content represented by the proportion of Sicontained in the porous silicon material as an element excluding oxygenmay be 98% by mass or less, 97% by mass or less, or 96% by mass or less.In the porous silicon material, the content of Al is in a range of 15%by mass or less. In embodiments, the content of Al may be lower, whichis, in embodiments, 10% by mass or less, in embodiments, 6.5% by mass orless, and, in embodiments, 5.0% by mass or less. The content of Al inthe porous silicon material may be 0.1% by mass or more, 2% by mass ormore, or 3% by mass or more. The porous silicon material may contain, asa second element, one or more elements among Ca, Cu, Mg, Na, Sr, and Pin a range of 15% by mass or less. In embodiments, the content ofelements other than Si may be lower. The porous silicon material may becomposed of Si in a predetermined range and Al plus unavoidableimpurities as a remainder. The content of Si in a predetermined rangemay, for example, be in a range of 85% by mass or more and 99.9% by massor less.

In the porous silicon material, Al is on the surface of the skeletalsilicon. The Al may coat the entire surface of the skeletal silicon, ormay coat a part of the surface of the skeletal silicon. The Al on thesurface of the skeletal silicon may be metallic or may be an oxide. Theoxide may include, for example, an AlSi oxide, an AlFe oxide, a SiFeoxide, and an AlSiFe oxide, in addition to an Al oxide.

Electrode for Power Storage Device

The electrode for the power storage device of the present disclosureincludes the aforementioned porous silicon material as an electrodeactive material. The electrode can be either a positive electrode or anegative electrode based on the potential opposite to the potential ofthe active material. When lithium is used as a carrier, the electrodeis, in embodiments, a negative electrode. The electrode can be used, forexample, in a lithium ion secondary battery, a hybrid capacitor, and anair battery. The electrode for the power storage device may be anelectrode compressed, so that the average porosity of the porous siliconmaterial falls into a range of 5% by volume or more and 50% by volume orless. The electrode may be compressed when prepared, so that theporosity of the porous silicon material is reduced. Compared to theporous silicon material prepared as a material having a porosity in arange of 5% by volume or more and 50% by volume or less, a poroussilicon material that is prepared as a material having a porosity of 50%by volume or more and 95% or less and then compressed, so that theporosity falls into a range of 5% by volume or more and 50% by volume orless exhibits better charge and discharge characteristics due to theshape of voids and the like. For example, when porous silicon particlesare used as a negative electrode active material of a lithium ionsecondary battery, the smaller the pores, the more uniform the obtainedalloy when lithium ions take part in alloying. Therefore, stressconcentration is reduced, which makes it possible to prevent thedeterioration of the electrode. The average porosity of the compressedporous silicon material may be appropriately adjusted depending on thecharacteristics that the electrode for the power storage device needs tohave. For example, the average porosity of the compressed porous siliconmaterial may be 10% by volume or more or 20% by volume or more. Theaverage porosity of the compressed porous silicon material may be, forexample, 40% by volume or less or 30% by volume or less.

The electrode for the power storage device may be an electrode includinga current collector and the aforementioned porous silicon material thatis formed and fixed on the current collector. The electrode can beprepared by a step of mixing the porous silicon material with aconductive material or a binder and a solvent as needed and making themixture into a paste or a step of mixing the porous silicon materialwith a conductive material or a binder as needed and bonding the mixtureto a current collector under pressure. In embodiments, the content ofthe porous silicon material in the electrode may be higher. The contentof the porous silicon material in the electrode is, in embodiments, 70%by mass or more, in embodiments, 80% by mass or more, and, inembodiments, 85% by mass or more. The conductive material is notparticularly limited as long as it is an electron conducting materialthat does not adversely affect the battery performance. For example,among graphite such as natural graphite (flaky graphite or scale-likegraphite) or artificial graphite, acetylene black, carbon black, Ketjenblack, carbon whisker, needle cokes, carbon fiber, and a metal (such ascopper, nickel, aluminum, silver, or gold), one kind of material or amixture of two or more kinds of these materials can be used. The binderis a material that functions to bind active material particles andconductive material particles together. For example, among afluorine-containing resin, such as polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), or fluororubber, a thermoplastic resin,such as polypropylene or polyethylene, ethylene propylene diene rubber(EPDM), sulfonated EPDM rubber, and natural butyl rubber (NBR), one kindof material or a mixture of two or more kinds of these materials can beused. It is also possible to use a cellulose-based binder that is anaqueous binder, an aqueous dispersion of styrene-butadiene rubber (SBR),and the like. As the solvent, for example, an organic solvent, such asN-methylpyrrolidone, dimethylformamide, dimethylacetamide,methylethylketone, cyclohexanone, methyl acetate, methyl acrylate,diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, ortetrahydrofuran, can be used. A dispersant, a thickener, or the like maybe added to water, and the active material may be made into a slurry byusing a latex, such as SBR. Examples of coating methods include rollercoating with an applicator roll, screen coating, a doctor blade method,spin coating, and bar coating. Any of these can be used to obtain anythickness and shape. The current collector may be appropriately selecteddepending on the potential of the active material. For example, inaddition to aluminum, titanium, stainless steel, nickel, iron, copper,calcined carbon, a conductive polymer, and conductive glass, for thepurpose of improving adhesiveness, conductivity, and oxidationresistance, a current collector prepared by treating the surface ofaluminum or copper with carbon, nickel, titanium, or silver can be used.The surface can be oxidized as well. The current collector may be in theform of, for example, a foil, film, sheet, net, punched or expandedmaterial, lath, porous material, foamed material, or material formed ofa fiber assembly. The thickness of the current collector to be used is,for example, 1 μm to 500 μm. The amount of active material complex to beformed may be appropriately set depending on the desired performancethat the power storage device needs to demonstrate.

In the electrode, as long as a low confining pressure and a capacityretention rate can be simultaneously achieved, the electrode activematerial may contain active materials other than the porous siliconmaterial, in addition to the porous silicon material. For example, theelectrode active material may contain a carbonaceous material orLi₄Ti₅O₁₂. Here, from the viewpoint of further increasing the batterycapacity, on the assumption that the total amount of the electrodeactive material is 100% by mass, the proportion of the porous siliconmaterial is, for example, 50% by mass or more and, in embodiments, 90%by mass or more.

Power Storage Device

The power storage device of the present disclosure includes an electrodehaving the aforementioned porous silicon material. The power storagedevice may include a positive electrode, a negative electrode, and anion conducting medium that is interposed between the positive electrodeand the negative electrode and conducts carrier ions. The porous siliconmaterial can be used as a negative electrode active material. The powerstorage device may be any one of a lithium ion secondary battery, ahybrid capacitor, an air battery, and the like. In the positiveelectrode, as a positive electrode active material, it is possible touse a sulfide containing a transition metal element, an oxide containinglithium and a transition metal element, and the like. Specifically, itis possible to use a transition metal sulfide, such as TiS₂, TiS₃, MoS₃,or FeS₂, a lithium-manganese composite oxide represented by a basiccomposition formula Li_((1−x))MnO₂ (satisfying 0<x<1 or the like, thesame shall applied hereinafter) or Li_((1−x))Mn₂O₄, a lithium-cobaltcomposite oxide represented by a basic composition formulaLi_((1−x))CoO₂ or the like, a lithium-nickel composite oxide representedby a basic composition formula Li_((1−x))NiO₂ or the like, alithium-nickel-cobalt-manganese composite oxide represented by a basiccomposition formula Li_((1−x))Ni_(a)Co_(b)Mn_(c)O₂ (a+b+c=1) or thelike, a lithium-vanadium composite oxide represented by a basiccomposition formula LiV₂O₃ or the like, a transition metal oxiderepresented by a basic composition formula V₂O₅ or the like, and thelike. Among these, a lithium transition metal composite oxide, such asLiCoO₂, LiNiO₂, LiMnO₂, or Li_((1−x))Ni_(1/3)Co_(1/3)Mn_(1/3)O₂, isused, in embodiments. Note that “basic composition formula” means thatthe positive electrode active material may also contain other elements,such as Al and Mg. Alternatively, the positive electrode active materialmay be a carbonaceous material used in a capacitor, a lithium ioncapacitor, or the like. Examples of the carbonaceous material includeactivated carbons, cokes, glassy carbons, graphites, non-graphitizablecarbons, pyrolytic carbons, carbon fibers, carbon nanotubes, andpolyacenes. Among these, activated carbons having a high specificsurface area may be used, in embodiments. The specific surface area ofthe activated carbon as a carbonaceous material is, in embodiments,1,000 m²/g or more, and, in embodiments, 1,500 m²/g or more. In a casewhere the specific surface area is 1,000 m²/g or more, the dischargecapacity can be further increased. In view of ease of preparation, thespecific surface area of the activated carbon is, in embodiments, 3,000m²/g or less, and, in embodiments, 2,000 m²/g or less. As the conductivematerial, the binder, the solvent, the current collector, and the liketo be used for the positive electrode, those exemplified above regardingthe aforementioned electrode can be appropriately used.

As the ion conducting medium, a non-aqueous electrolytic solutioncontaining a supporting salt, a non-aqueous gel electrolytic solution,or the like can be used. Examples of solvents for the non-aqueouselectrolytic solution include carbonates, esters, ethers, nitriles,furans, sulfolanes, and dioxolanes. These may be used independently orused by being mixed together. Specifically, examples of the carbonatesinclude cyclic carbonates, such as ethylene carbonate, propylenecarbonate, vinylene carbonate, butylene carbonate, and chloroethylenecarbonate, chain-like carbonates, such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate,methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propylcarbonate, cyclic esters, such as γ-butyl lactone and γ-valerolactone,chain-like esters, such as methyl formate, methyl acetate, ethylacetate, and methyl butyrate, ethers, such as dimethoxyethane,ethoxymethoxyethane, and diethoxyethane, nitriles, such as acetonitrileand benzonitrile, furans, such as tetrahydrofuran and methyltetrahydrofuran, sulfolanes, such as sulfolane and tetramethylsulfolane,and dioxolanes, such as 1,3-dioxolane and methyl dioxolane. Among these,a combination of cyclic carbonates and chain-like carbonates may beused, in embodiments. This combination not only makes it possible toobtain excellent cycle characteristics that indicate the characteristicsof a battery repeatedly charged and discharged, and also makes itpossible to allow the viscosity of the electrolytic solution, theelectric capacity of the obtained battery, and the battery output to bebalanced. Examples of the supporting salt include LiPF₆, LiBF₄, LiAsF₆,LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, LiSbF₆, LiSiF₆, LiAlF₄, LiSCN,LiClO₄, LiCl, LiF, LiBr, LiI, and LiAlCl₄. In view of electricalcharacteristics, among the supporting salts, one kind of salt selectedfrom the group consisting of an inorganic salt, such as LiPF₆, LiBF₄,LiAsF₆, or LiClO₄, and an organic salt, such as LiCF₃SO₃, LiN(CF₃SO₂)₂,or LiC(CF₃SO₂)₃ may be used, in embodiments, or a combination of two ormore kinds of salts selected from the same group may be used, inembodiments. The concentration of the supporting salt in the non-aqueouselectrolytic solution is, in embodiments, 0.1 mol/L or more and 5 mol/Lor less, and, in embodiments, 0.5 mol/L or more and 2 mol/L or less. Ina case where the concentration of the supporting salt dissolved is 0.1mol/L or more, a sufficient current density can be obtained. In a casewhere the concentration of the supporting salt dissolved is 5 mol/L orless, the electrolytic solution can be more stabilized. A flameretardant, such as a phosphorus-based flame retardant or a halogen-basedflame retardant, may be added to the non-aqueous electrolytic solution.

Instead of the liquid ion conducting medium, a solid ion conductivepolymer can be used as the ion conducting medium. As the ion conductivepolymer, for example, it is possible to use a polymer gel composed of apolymer, such as acrylonitrile, ethylene oxide, propylene oxide, methylmethacrylate, vinyl acetate, vinylpyrrolidone, or vinylidene fluoride,and a supporting salt. An ion conductive polymer and a non-aqueouselectrolytic solution can be used in combination. As the ion conductingmedium, in addition to the ion conductive polymer, it is also possibleto use an inorganic solid electrolyte, a mixed material of an organicpolymer electrolyte and an inorganic solid electrolyte, inorganic solidpowder bound by an organic binder, and the like.

The power storage device may include a separator between the negativeelectrode and the positive electrode. The separator is not particularlylimited as long as it has a composition durable in the range of use ofthe lithium secondary battery. Examples of the separator include apolymeric nonwoven fabric, such as a nonwoven fabric made ofpolypropylene or a nonwoven fabric made of polyphenylene sulfide, and athin microporous film of an olefin-based resin, such as polyethylene orpolypropylene. Each of these may be used alone, or a plurality of thesemay be used by being mixed together.

The shape of the power storage device is not particularly limited, andexamples thereof include a coin shape, a button shape, a sheet shape, alaminate shape, a cylindrical shape, a flat shape, and an angular shape.The power storage device may be applied to a large-sized power storagedevice used in a battery electric vehicle or the like. FIG. 2 is a viewillustrating an example of the structure of a power storage device 10.The power storage device 10 has a positive electrode 12, a negativeelectrode 15, and an ion conducting medium 18. The positive electrode 12has a positive electrode active material 13 and a current collector 14.The negative electrode 15 has a negative electrode active material 16and a current collector 17. The negative electrode active material 16 isa porous silicon material 21 described above and has voids 23.

All-Solid-State Lithium-Ion Secondary Battery

The power storage device is, in embodiments, an all-solid-statelithium-ion secondary battery. The all-solid-state battery may beselected, in embodiments, because the battery makes it possible tofurther suppress performance change caused by an electrolytic solutionand to further enhance safety. The all-solid-state lithium-ion secondarybattery may include a positive electrode containing a positive electrodeactive material, a negative electrode having the aforementioned poroussilicon material as a negative electrode active material, and a solidelectrolyte that is interposed between the positive electrode and thenegative electrode and conducts lithium ions. As the positive electrode,any of the materials listed above regarding the aforementioned powerstorage device can be used. As the negative electrode, a negativeelectrode having the aforementioned porous silicon material can be used.

The solid electrolyte may be, for example, a garnet-type oxidecontaining at least Li, La, and Zr. The solid electrolyte may have abasic composition represented by Li_(0.7+x−y)(La_(3−x), A_(x))(Zr_(2−y), T_(y))O₁₂. Here, A is one or more kinds of elements betweenSr and Ca, T is one or more kinds of elements between Nb and Ta, xsatisfies 0<x≤1.0, and y satisfies 0<y<0.75. Alternatively, the solidelectrolyte may be a garnet-type oxide having a basic compositionrepresented by (Li_(7−3z+x−y)M_(z))(La_(3−x)A_(x))(Zr_(2−y)T_(y))O₁₂ or(Li_(7−3z+x−y)M_(z))(La_(3−x)A_(x))(Y_(2−y)T_(y))O₁₂. In the formulas,the element M may be one or more elements between Al and Ga, the elementA may be one or more elements between Ca and Sr, T may be 1 or moreelements between Nb and Ta, z may satisfy 0≤z≤0.2, x may satisfy0≤x≤0.2, and y may satisfy 0≤y≤2. In the basic composition formula, zsatisfies 0.05≤z≤0.1, in embodiments. In the basic composition formula,x satisfies 0.05≤x≤0.1, in embodiments. In the basic compositionformula, y satisfies 0.1≤y≤0.8, in embodiments. In a case where each ofx, y, and z is in the above range, more suitable ion conductivity can beobtained.

Examples of the solid electrolyte also include Li₃N, Li₁₄Zn(GeO₄)₄called LISICON, Li_(3.25)Ge_(0.25)P_(0.75)S₄ as a sulfide,La_(0.5)Li_(0.5)TiO₃ as a Perobskite-type solid electrolyte,(La_(2/3)Li_(3x)□_(1/3−2x))TiO₃ (□: atomic vacancy), Li₇La₃Zr₂O₁₂ as agarnet-type solid electrolyte, LiTi₂(PO₄)₃ called NASICON-type solidelectrolyte, and Li_(1.3)M_(0.3)Ti_(1.7)(PO₃)₄ (M=Sc, Al). Examples ofthe solid electrolyte also include Li₇P₃S₁₁ obtained from glass having acomposition of 80Li₂S·20P₂S₅ (mol %), which is glass ceramics, andLi₁₀Ge₂PS₂ as a sulfide-based electrolyte that is a substance havinghigh electric conductivity. Examples of glass-based inorganic solidelectrolytes include Li₂S—SiS₂, Li₂S—SiS₂—LiI, Li₂S—SiS₂—Li₃PO₄,Li₂S—SiS₂—Li₄SiO₄, Li₂S—P₂S₅, Li₃PO₄—Li₄SiO₄, Li₃BO₄—Li₄SiO₄, and anelectrolyte composed of SiO₂, GeO₂, B₂O₃, or P₂O₅ as a glass-basedsubstance and Li₂O as a network modifying substance. Examples ofthio-LISICON solid electrolytes include a Li₂S—GeS₂ system, aLi₂S—GeS₂—ZnS system, a Li₂S—Ga₂S₂ system, a Li₂S—GeS₂—Ga₂S₃ system, aLi₂S—GeS₂—P₂S₅ system, a Li₂S—GeS₂—SbS₅ system, a Li₂S—GeS₂—Al₂S₃system, a Li₂S—SiS₂ system, a Li₂S—P₂S₅ system, a Li₂S—Al₂S₃ system, aLiS—SiS₂—Al₂S₃ system, and a Li₂S—SiS₂—P₂S₅ system. These solidelectrolytes may be formed into a plate and disposed between a positiveelectrode and a negative electrode.

The all-solid-state lithium-ion secondary battery may include arestraining member that restrains a laminate composed of a positiveelectrode, a solid electrolyte, and a negative electrode laminated oneach other in the lamination direction. The restraining member mayinclude, for example, a pair of plate-shaped portions that presses theinterposed laminate from both ends of the laminate in the laminationdirection, a rod-shaped portion that connects the pair of plate-shapedportions, and an adjusting portion that is connected to the rod-shapedportion and adjusts the space between the pair of plate-shaped portionsby a screw structure.

Manufacturing Method of Electrode for Power Storage Device

The manufacturing method of an electrode for a power storage deviceincludes a pressing step of compressing a porous silicon materialobtained by the aforementioned manufacturing method of a porous siliconmaterial as an electrode active material, so that the average porosityof the porous silicon material falls into a range of 5% by volume ormore and 50% by volume or less. The pressing step makes it possible toreduce porosity while maintaining a suitable pore shape and to furtherincrease energy density. In the pressing step, the porous siliconmaterial may be compressed, so that the average porosity thereof fallsinto a range of 10% by volume or more or 20% by volume or more, and arange of 40% by volume or less or 30% by volume or less. The averageporosity of the compressed porous silicon material may be appropriatelyadjusted depending on the characteristics that the electrode for thepower storage device needs to have. In the pressing step, for example,the electrode may be pressed under a pressure in a range of 2 MPa ormore and 20 MPa or less. The pressing step may be a step of performing atreatment of mixing the porous silicon material with a conductivematerial or a binder and a solvent as needed and making the mixture intoa paste or a treatment of mixing the porous silicon material with aconductive material or a binder as needed and bonding the mixture to acurrent collector under pressure. As for the amount of the poroussilicon material to be mixed in and the like, what are described aboveregarding the electrode for the power storage device can beappropriately employed.

Manufacturing Method of Power Storage Device

The manufacturing method of a power storage device may be a method ofusing an electrode for a power storage device obtained by theaforementioned manufacturing method of an electrode for a power storagedevice as a negative electrode, disposing a positive electrode and thenegative electrode to make the electrodes face each other, andinterposing an ion conducting medium conducting lithium ions between thepositive electrode and the negative electrode. A separator may beinterposed between the positive electrode and the negative electrode. Asthe positive electrode, the negative electrode, the ion conductingmedium, and the separator, it is possible to appropriately use any ofthose exemplified above for the aforementioned power storage device.

Manufacturing Method of All-Solid-State Lithium-Ion Secondary Battery

The manufacturing method of an all-solid-state lithium-ion secondarybattery includes a laminate preparation step of preparing a laminatecomposed of a positive electrode, a solid electrolyte conducting lithiumions, and a negative electrode laminated on each other, by using anelectrode for a power storage device obtained by the aforementionedmanufacturing method of an electrode for a power storage device as anegative electrode. As the positive electrode, the negative electrode,and the solid electrolyte used in the manufacturing method, it ispossible to appropriately use any of those exemplified above for theaforementioned all-solid-state lithium-ion secondary battery. Thethickness or size of the positive electrode, the negative electrode, andthe solid electrolyte used in the manufacturing method can beappropriately selected depending on the desired battery characteristics.In the manufacturing method, the pressing step in the manufacturingmethod of an electrode for a power storage device may be performed inthe laminate preparation step. That is, in the step of pressing thelaminate composed of the positive electrode, the solid electrolyte, andthe negative electrode laminated on each other, the negative electrodemay be pressed as well.

The manufacturing method of an all-solid-state lithium-ion secondarybattery may further include a laminate restraining step of restrainingthe prepared laminate with a restraining member in the laminationdirection. The restraining pressure for the laminate is, in embodiments,in a range of, for example, 2 MPa or more and 20 MPa or less.

As being specifically described above, the present disclosure canfurther improve the electrochemical characteristics of materialscontaining Si. The reason why such an effect is obtained is presumed asfollows. For example, in a lithium ion secondary battery, a siliconelectrode has a theoretical capacity of 4,199 mAh/g, which is about 10times higher than theoretical capacity, 372 mAh/g, of general graphite.Therefore, the silicon electrode is expected to further increasecapacity and energy density. Meanwhile, silicon having absorbed lithiumions is Li_(4.4)Si, which has a volume having expanded about 4 times thevolume of silicon not yet absorbing lithium. According to the presentdisclosure, a substance other than silicon that is mainly Al andincorporated into a silicon alloy is selectively removed by beingdissolved, which makes it possible to easily produce a porous siliconmaterial having a small pore size and a high porosity. When the poroussilicon material having a small pore size and a high porosity is used ina power storage device, such as a lithium ion secondary battery, theexpansion and contraction of volume are significantly mitigated.Accordingly, for example, charge and discharge characteristics, such ascharge and discharge cycle characteristics, can be improved, which makesit possible to easily obtain a high-performance power storage device. Inthe porous silicon material, other elements, such as Al, are diffused onthe surface. Therefore, the porous silicon material has a purer siliconskeleton, which can further improve electrochemical characteristics,such as charge and discharge capacities.

Note that the present disclosure is not limited to the embodimentsdescribed above. It goes without saying that the present disclosure canbe embodied in various aspects as long as the aspects are within thetechnical scope of the present disclosure.

Hereinafter, the cases where the porous silicon and the power storagedevice of the present disclosure are manufactured in specific ways willbe described as experimental examples. Experimental Examples 8 to 11 areexamples of the present disclosure, Experimental Example 12 is acomparative example, and Experimental Examples 1 to 7 are referenceexamples. First, a porous silicon material in an Al—Si eutecticcomposition was examined (Experimental Examples 1 to 7).

EXPERIMENTAL EXAMPLE 1

Al lumps (87% by mass) having sides of 10 mm and 13% by mass of Si lumpswere weighed, mixed together, and dissolved in an Ar inert atmosphere bya high-frequency heating method, thereby obtaining a molten alloy. Themolten alloy was subjected to gas atomization using an Ar inert gas,thereby obtaining AlSi alloy powder having an average particle size of 8μm (particle forming step). The alloy powder was AlSi alloy powder thatis an Al—Si eutectic composition (see FIG. 1 ), and composed of aeutectic Si phase and an Al phase. As a result of analyzing the obtainedpowder by X-ray diffraction, the presence of an Al phase and a Si phaseas crystalline phases was confirmed. Next, the obtained alloy powder wasadded to 3 mol/L hydrochloric acid diluted with pure water, stirred atroom temperature of 25° C. for 1 hour, then filtered while beingthoroughly washed, and dried in a vacuum drier at 30° C. for 2 hours(pore forming step). In this way, a porous silicon material ofExperimental Example 1 was prepared.

EXPERIMENTAL EXAMPLE 2

A negative electrode active material of Experimental Example 2 wasmanufactured in the same manner as in Experimental Example 1, exceptthat the amounts of Al and Si used were changed to 82% by mass and 18%by mass respectively. The alloy powder in Experimental Example 2 wasAlSi alloy powder that is an Al—Si hypereutectic composition andcomposed of primary Si, a eutectic Si phase, and an Al phase. An acidtreatment was carried out under the same conditions as in ExperimentalExample 1, thereby obtaining a negative electrode active material ofExperimental Example 2.

EXPERIMENTAL EXAMPLE 3

A negative electrode active material of Experimental Example 3 wasmanufactured in the same manner as in Experimental Example 1, exceptthat the amounts of Al and Si used were changed to 73% by mass and 27%by mass respectively. The alloy powder in Experimental Example 2 wasAlSi alloy powder that is an Al—Si hypereutectic composition andcomposed of primary Si, a eutectic Si phase, and an Al phase. An acidtreatment was carried out under the same conditions as in ExperimentalExample 1, thereby obtaining a negative electrode active material ofExperimental Example 3.

EXPERIMENTAL EXAMPLE 4

A negative electrode active material of Experimental Example 4 wasmanufactured in the same manner as in Experimental Example 1, exceptthat the amounts of Al and Si used were changed to 90% by mass and 10%by mass respectively. The alloy powder in Experimental Example 4was AlSialloy powder that is an Al—Si hypoeutectic composition and composed ofprimary Al, a eutectic Si phase, and an Al phase. An acid treatment wascarried out under the same conditions as in Experimental Example 1,thereby obtaining a negative electrode active material of ExperimentalExample 4.

EXPERIMENTAL EXAMPLE 5

A negative electrode active material of Experimental Example 5 wasmanufactured in the same manner as in Experimental Example 1, exceptthat the amounts of Al, Si, and Cu used were changed to 87% by mass, 10%by mass, and 3% by mass respectively. The alloy powder in ExperimentalExample 5 was AlSiCu alloy powder that is composed of primary Al, aeutectic Si phase, and an Al₂Cu phase. An acid treatment was carried outunder the same conditions as in Experimental Example 1, therebyobtaining a negative electrode active material of Experimental Example5.

EXPERIMENTAL EXAMPLE 6

A negative electrode active material of Experimental Example 6 wasmanufactured in the same manner as in Experimental Example 1, exceptthat the amounts of Al and Si used were changed to 43% by mass and 57%by mass respectively. The alloy powder in Experimental Example 2 wasAlSi alloy powder that is an Al—Si hypereutectic composition andcomposed of primary Si, a eutectic Si phase, and an Al phase. An acidtreatment was carried out under the same conditions as in ExperimentalExample 1, thereby obtaining a negative electrode active material ofExperimental Example 6.

EXPERIMENTAL EXAMPLE 7

Si powder having an average particle size of 5 μm was used as a negativeelectrode active material of Experimental Example 7.

Measurement of Physical Properties of Porous Silicon Material

The porous silicon powder having undergone the acid treatment wasdissolved by HF and HNO₃, and the amount of Al in porous silicon wasmeasured by ICP optical emission spectroscopy (ICP-OES, PS3520U VDDIIIImanufactured by Hitachi High-Tech Science Corporation.). In addition,elemental analysis was performed by observation with a scanning electronmicroscope (SEM, S-4300 manufactured by Hitachi High-Tech ScienceCorporation.) and energy dispersive X-ray analysis (EDAX, S-4300manufactured by Hitachi High-Tech Science Corporation.). The poredistribution was measured with a mercury porosimeter (POWERMASTER60GTmanufactured by Quantachrome Instruments).

Results of Physical Property Measurement and Review

FIG. 3 is an SEM image of the Al—Si alloy observed after gas atomizationin Experimental Example 1. As shown in FIG. 3 , a structure wasconfirmed in which eutectic Si exists between large primary Al. FIGS. 4Ato 4D are SEM images and an EDAX mapping images of the porous siliconmaterial of Experimental Example 1. FIG. 4A is a full image, FIGS. 4Band 4C are enlarged images, and FIG. 4D is an EDAX image. InExperimental Example 1, it was confirmed that Si takes up 13% by mass inthe raw material composition and is a structure containing skeletalsilicon that maintains a particle shape and has a three-dimensionalnetwork structure having voids on the inside of the particle as shown inFIG. 4B. From the EDAX image, it was revealed that the structure iscomposed of Si, although extremely small amounts of Al and O weredetected. It was confirmed that most of the primary Al is removed by thepore forming step as an acid treatment at room temperature. The Alconcentration determined by ICP-OES after the pore forming treatment was4.2% by mass. From the result, it was considered that the composition ofthe raw material powder tells that most of the aluminum is eluted andporous silicon is formed. FIG. 5 is a pore distribution curve of theporous silicon material of Experimental Example 1. In mercuryporosimeter, sometimes the holes between particles are included in themeasurement result. Therefore, the alloy particles not yet beingsubjected to the pore forming treatment and the porous silicon materialobtained after the pore forming treatment were measured, and thedifference therebetween was adopted as a pore volume. In FIG. 5 , thedifference is represented by a shaded portion. As shown in FIG. 5 , thepore size is 1 μm or less, which is consistent with the size of voidsconfirmed by the SEM image. As a result of measurement with the mercuryporosimeter, it was confirmed that the porous silicon material ofExperimental Example 1 has a pore distribution of 50 nm to 500 nm inwhich a pore size of around 200 nm takes up the highest proportion, andhas a porosity of 76% by volume.

The composition ratio of raw materials (% by mass), the amount ofresidual Al after acid treatment (% by mass), and the porosity (% byvolume) of Experimental Examples 1 to 7 are summarized in Table 1. Theamount of Al is a measurement result obtained by ICP-OES, and theporosity is a value excluding holes between particles. As shown in Table1, it was found that the amount of residual Al after the acid treatmentin Experimental Examples 1 to 7 is 3.2% to 6.2% by mass, and most of Alis eluted by the pore forming treatment which is an acid treatment.While the porosity of Experimental Examples 1 to 5 was 62% to 86% byvolume, the porosity of Experimental Example 6 was 33% by volume whichis a low porosity.

TABLE 1 Composition ratio Amount of Al of raw material remaining afteracid Porosity (% by mass) treatment % by Si AI Cu % by mass volumeExample 1 13 87 — 4.2 76 Example 2 18 82 — 3.6 68 Example 3 27 73 — 3.262 Example 4 10 90 — 6.2 86 Example 5 10 87 3 5.1 84 Example 6 57 43 —3.8 33 Example 7 — — — — —

Preparation of Lithium Secondary Battery Using a Non-AqueousElectrolytic Solution

Each of the silicon materials (82% by mass) of Experimental Examples 1to 7 as a negative electrode active material, 6% by mass of acetyleneblack having an average particle size of 2 μm as a conductive material,and 12% by mass of polyimide as a binder were weighed, mixed together,mixed with N-methylpyrrolidone, and then stirred, thereby preparing aslurry as a negative electrode mixture. The slurry was then applied to acopper foil having a thickness of 12 μm, dried, and rolled, therebypreparing a negative electrode having a thickness of 50 μm. It wasassumed that the rolling might reduce the porosity of the porous siliconmaterial to about 50% by volume while allowing the porous siliconmaterial to maintain the three-dimensional network structure. Theprepared negative electrode was punched into circles having a diameterof 16 mm, a porous polyethylene separator was interposed between thenegative electrodes, and metallic lithium was stacked thereon as acounter electrode, thereby forming a laminate. Subsequently, anelectrolytic solution obtained by adding LiPF₆ at a concentration of 1mol/L to a mixed solvent obtained by mixing together ethylene carbonate(EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) at a volumeratio of 3:4:3 was injected into the aforementioned laminate, therebymanufacturing a lithium secondary battery which is a Tomcell-type smallbattery cell. The obtained lithium secondary battery was repeatedlycharged and discharged 10 cycles at a current density of 0.2 C and abattery voltage in a range of 0 V to 1.5 V.

Characteristics of Lithium Secondary Battery Using Non-AqueousElectrolytic Solution

The initial discharge capacity (mAh/g) of Experimental Examples 1 to 7,the discharge capacity (mAh/g) of the same examples after 10 cycles, andthe capacity retention rate (%) of the same examples are summarized inTable 2. The capacity retention rate was calculated from FormulaQ₁₀/Q₁×100 where Q₁ represents a discharge capacity in the first cycleand Q₁₀ represents the discharge capacity in the tenth cycle. As shownin Table 2, the capacity retention rate in Experimental Example 7 was26% that is a low rate, which leaded to the assumption that siliconparticles having no voids could not absorb the volume change and mightcause problems in the electrode. Even in Experimental Example 6 having aporosity of 33% by volume that is a low porosity, the capacity retentionrate was less than 70%, which tells that the capacity retention rate isinsufficient. On the other hand, the lithium secondary batteries ofExperimental Examples 1 to 5 had a capacity retention rate of 84% to 93%that is an excellent value, which tells that the electrodes are stable.In this way, as shown in Experimental Examples 1 to 5, it was revealedthat in a case where a porous silicon material having a porosity of 60%by volume or more is compressed and thus the porosity falls into a rangeof 5% by volume or more and 50% by volume or less, the capacityretention rate can be particularly increased.

TABLE 2 Non-aqueous Initial discharge Discharge capacity Capacityelectrolytic solution capacity after 10 cycles retention rate¹⁾ cellmAh/g mAh/g % Example 1 1980 1849 93.4 Example 2 1845 1697 92.0 Example3 1725 1451 84.1 Example 4 1824 1689 92.6 Example 5 1849 1725 93.3Example 6 1876 1276 68.0 Example 7 1937 494.8 25.5 ¹⁾Measured 10 cyclesat 0.2 C in range of 0 V to 1.5 V and calculated from Q₁₀/Q₁ × 100 whereQ₁ represents initial capacity (mAh/g) and Q₁₀ represents capacity(mAh/g) in 10th cycle

Next, a porous silicon material that was subjected to a heat treatmentafter the pore forming treatment was examined.

EXPERIMENTAL EXAMPLE 8

Al lumps (88% by mass) having sides of 10 mm and 12% by mass of Si lumpswere weighed, mixed together, and dissolved in an Ar inert atmosphere bya high-frequency heating method, thereby obtaining a molten alloy. Themolten alloy was subjected to gas atomization using an Ar inert gas,thereby obtaining AlSi alloy powder having an average particle size of 8μm (particle forming step). The composition of the atomized powder canbe represented by Al-12.4% by mass of Si-0.14% by mass of Fe, and thestructure thereof was composed of primary Al and eutectic Si. Next, theobtained alloy powder was added to 3 mol/L hydrochloric acid dilutedwith pure water, stirred at room temperature of 25° C. for 1 hour, thenfiltered while being thoroughly washed, and dried in a vacuum drier at30° C. for 2 hours. (pore forming step). A treatment for eluting theprimary Al was performed, thereby obtaining a porous material (poroussilicon particles as a precursor). The obtained porous material wassubjected to a heat treatment in an Ar atmosphere under the conditionsof 1,000° C. and 2 hours (heat treatment step), and the obtainedstrengthened porous silicon powder was used as a porous silicon materialof Experimental Example 8. In the heat treatment step, Al present as asolid solution in eutectic Si forming the skeleton can be diffused andremoved to the surface of the porous material, and an oxide contributingto the strengthening of the skeleton can be formed.

EXPERIMENTAL EXAMPLE 9

A porous silicon material of Experimental Example 9 was prepared in thesame manner as in Experimental Example 8 described above, except thatthe amounts of Al and Si used were changed to 83% by mass and 17% bymass respectively. The composition of the atomized powder can berepresented by Al-17.4% by mass of Si-0.12% by mass of Fe, and thestructure thereof was composed of primary Al and eutectic Si. An acidtreatment and a heat treatment were performed under the same conditionsas in Experimental Example 8, thereby obtaining a porous siliconmaterial of Experimental Example 9.

EXPERIMENTAL EXAMPLE 10

A porous silicon material of Experimental Example 10 was prepared in thesame manner as in Experimental Example 8 described above, except thatthe amounts of Al and Si used were changed to 81% by mass and 19% bymass respectively. The composition of the atomized powder can berepresented by Al-19.4% by mass of Si-0.13% by mass of Fe, and thestructure thereof was composed of primary Al, eutectic Si, and primarySi. An acid treatment and a heat treatment were performed under the sameconditions as in Experimental Example 8, thereby obtaining a poroussilicon material of Experimental Example 10.

EXPERIMENTAL EXAMPLE 11

A porous silicon material of Experimental Example 11 was prepared in thesame manner as in Experimental Example 8 described above, except thatthe amounts of Al and Si used were changed to 74% by mass and 26% bymass respectively. The composition of the atomized powder can berepresented by Al-25.5% by mass of Si-0.14% by mass of Fe, and thestructure thereof was composed of primary Al, eutectic Si, and primarySi. An acid treatment and a heat treatment were performed under the sameconditions as in Experimental Example 8, thereby obtaining a poroussilicon material of Experimental Example 11.

EXPERIMENTAL EXAMPLE 12

Si powder having an average particle size of 5 μm was used as a siliconmaterial of Experimental Example 12.

Measurement of Physical Properties of Porous Silicon Material

The porous silicon powder having undergone the acid treatment wasdissolved by HF and HNO₃, and the amount of Al in porous silicon wasmeasured by ICP optical emission spectroscopy (ICP-OES, PS3520U VDDIIIImanufactured by Hitachi High-Tech Science Corporation.). In addition,elemental analysis was performed by observation with a scanning electronmicroscope (SEM, S-4300 manufactured by Hitachi High-Tech ScienceCorporation.) and energy dispersive X-ray analysis (EDAX, S-4300manufactured by Hitachi High-Tech Science Corporation.). The poredistribution was measured with a mercury porosimeter (POWERMASTER6OGTmanufactured by Quantachrome Instruments).

Results and Review

Table 3 shows the raw material composition (at %) of ExperimentalExamples 8 to 12, the porosity (% by volume) of silicon materials afterthe pore forming treatment (acid treatment) and the heat treatment at1,000° C., the amount (% by mass) of Al solid solution after the acidtreatment (before the heat treatment), the lattice constant a_(Si) (Å)after the acid treatment, and the lattice constant a_(Si) (Å) after thepore forming treatment and the heat treatment. The porosity isrepresented by a value determined by the mercury intrusion porosimeter.The amount of Al solid solution is represented by a value determined byICP analysis. As shown in Table 3, the porous silicon materials havingundergone the heat treatment in Experimental Examples 8 to 11 have aporosity or of 50% by volume or more and may have a porosity of 60% byvolume or more, which tells that porous silicon materials could beformed. It was assumed that the amount of residual Al in the poroussilicon materials may be in a range of 3% to 5% by mass. As for thelattice constant a_(Si) of Si having undergone the acid treatment andnot yet being subjected to the heat treatment, while Si in ExperimentalExample 12 has a_(Si) of 5.430 (Å), Si in Experimental Examples 8 to 11has a_(Si) of 5.440 (Å) or more, which tells that Al is present as asolid solution in Si. As for the lattice constant a_(Si) of Si havingundergone the acid treatment and the heat treatment, Si in ExperimentalExamples 8 to 11 has a_(Si) of 5.435 (Å) or less which is the same asa_(Si) of Si in Experimental Example 12. From the result, it was assumedthat in Experimental Examples 8 to 11, by the heat treatment at 1,000°C., Al present as a solid solution in Si might diffuse to the surfaceside, which might lead to the transition to a silicon skeleton in whichno Al solid solution is present.

TABLE 3 Silicon material Lat- tice con- stant A- meas- Lattice mountured constant Porosity of before after after solid heat acid acid solu-treat- treat- treat- tion ment ment ment Al and and at after heat heatRaw material 1,000° C. acid treat- treat- Elemental treat- treat- mentment composition ment¹⁾ ment²⁾ is per- at (at %) % by % by formed 1,000°C. Si Al Fe volume mass Å Å Example 12.4 Bal. 0.15 76 4.1 5.450 5.428 8Example 17.4 Bal. 0.12 69 3.9 5.448 5.427 9 Example 19.4 Bal. 0.13 663.6 5.447 5.431 10 Example 25.5 Bal. 0.14 65 3.4 5.445 5.432 11 ExampleBal. — — — — 5.430 5.430 12 ¹⁾Measurement result obtained by mercuryintrusion porosimeter ²⁾Determination result of ICP analysis

FIG. 6 is an SEM image of the porous silicon material of ExperimentalExample 8. FIG. 6 is an SEM image of a porous silicon material obtainedby performing a heat treatment at 1,000° C. on a porous material in anAr atmosphere, the porous material being obtained by treating Al-12.6%by mass of Si atomized powder with a 3 mol/L hydrochloric acid solutionfor eluting Al. As shown in FIG. 6 , by the elution of primary Al in thesilicon compound with an acid, pores were formed, and porous siliconparticles were obtained. FIG. 7 is a pore distribution curve of theporous silicon material of Experimental Example 8. As shown in FIG. 7 ,after being subjected to the heating treatment at 1,000° C., the poroussilicon material had a pore distribution of several tens of nanometersto 11 μm where the peak of pore size was 200 nm.

Next, the change in crystal structure of the porous silicon materialcaused by the heat treatment temperature was investigated by X-raydiffraction. As a heat treatment, raw materials having the compositionof Experimental Example 8 were heated from 100° C. to 1,000° C. by 100°C. in an Ar atmosphere, and the obtained porous silicon material wasanalyzed by XRD. FIG. 8 is a diagram showing the relationship betweenheat treatment temperatures and XRD profiles in Experimental Example 8.From the measurement result of the XRD profiles, the lattice constanta_(Si) of Si was calculated by Rietveld analysis. FIG. 9 is a diagramshowing the relationship between heat treatment temperatures and alattice constant a_(Si) in Experimental Example 8. From the X-raydiffraction pattern shown in FIG. 8 , a broad peak derived fromamorphous phase was observed at around 2θ=20° to 25°. The amorphousphase is derived from SiO₂. SiO₂ is present in the porous siliconmaterial before the heat treatment. SiO₂ has approximately doubled interms of molar ratio after the heat treatment at a temperature of 600°C. or higher. A peak of Al was detected in a temperature range of theheat treatment temperature of 400° C. to 500° C. In the atomized powder,the content of Fe impurities was about 0.1% by mass. Therefore, at atemperature of 700° C. or higher, a peak of FeSi₂ was detected, whichtells that FeSi₂ was generated. As shown in FIG. 9 , from the change inthe lattice constant a_(Si) of Si caused by the heat treatmenttemperature, it was assumed that the lattice constant a_(Si) of Si notyet being subjected to the heat treatment might have a larger latticeconstant a_(Si) compared to a pure Si standard sample and might containAl as a solid solution. By the heat treatment, the lattice constantbegan to decrease at 300° C., and became substantially equal to thelattice constant of pure Si at 400° C. From the result, it was assumedthat Al present as a solid solution in the porous material might beprecipitated at a temperature of 400° C. or higher, Al present as asolid solution in Si might diffuse to the surface thereof and be removedfrom the inside of the silicon skeleton.

Next, the amount of Al present on the surface of the porous siliconmaterial having undergone a heat treatment in a temperature range of100° C. to 1,000° C. was measured. In this way, whether the amount of Alsolid solution in the silicon skeleton in the porous silicon material isreduced after the heat treatment was investigated. FIG. 10 is a diagramshowing the relationship between heat treatment temperatures and amountsof Al dissolved (% by mass) in Experimental Example 8. The amount of Alpresent on the surface was measured as follows. First, the porousmaterial having the composition of Experimental Example 8 was subjectedto a heat treatment at each temperature, and 10 mL of 6 mol/L nitricacid and 5 mL of 50% by volume hydrofluoric acid were added to each ofthe obtained porous silicon materials and dissolved by heating. Then, 10mL of 8 mol/L sulfuric acid was added thereto, the mixture was furtherheated to generate white smoke of sulfuric acid, and then the mixturewas diluted to 100 mL. In the acid treatment for ICP analysis, Si,Al₂O₃, and the like are not dissolved, and Al metal is dissolved. Byusing the solution, the amount of Al was quantified by an inductivelycoupled plasma emission spectrophotometer. In FIG. 10 , the ordinateshows the amount of Al dissolved (% by mass) with respect to the totalamount of the porous silicon material including Si and Al, the dashedline shows the amount of Al as a solid solution (4.1% by mass) in theporous silicon material not yet being subjected to the heat treatment,and the dotted line is a line corresponding to 90% by mass (3.7% bymass) of the amount represented by the dashed line. Al present as asolid solution in the porous silicon material began to be precipitatedon the surface of the silicon skeleton by the heat treatment at atemperature of 400° C. or higher, then kept increasing until thetemperature reached 700° C., and decreased again when the temperaturewas 800° C. or higher. It was revealed that in a case where the heattreatment is performed in an Ar atmosphere in a range of 400° C. to 800°C., Al present as a solid solution in the silicon skeleton is removedfrom the inside of the silicon skeleton and diffuses to the surface ofthe silicon skeleton, and particularly in a range of 600° C. to 800° C.,Al as a solid solution can be reduced by about 90%. It is expected thatat a temperature of 800° C. or higher, even in an Ar atmosphere, the Almetal may be oxidized, and Al oxide may be formed on the surface of thesilicon skeleton. It was assumed that accordingly, the Al metal mightnot be dissolved by the acid treatment for ICP analysis, and the amountof Al dissolved might be reduced.

From the result, it was revealed that performing a heat treatment at atemperature of 600° C. to 800° C. and then performing an acid treatmenton the porous silicon material makes it possible to reduce the amount ofAl in Si while maintaining the microstructure that can suppress volumeexpansion. To sum up the results of the crystal structure changeinvestigated by X-ray diffraction, it was assumed that the state of Alin the porous silicon material might change as shown in FIGS. 11A to 11Ddue to the heat treatment. FIGS. 11A to 11D are views illustrating howthe state of the porous silicon material changes by each treatment. In acase where an acid treatment is performed on the AlSi alloy powderobtained by the atomization treatment, solely the primary Al is eluted,and the residual eutectic Si forms a skeleton and turns into a porousmaterial (Si) (FIG. 11A). In a quenching atomization treatment, Alcontained in the eutectic Si is in a state of solid solution. In a casewhere Al present as a solid solution in Si is subjected to a heatingtreatment at a temperature of 300° C. or higher, phase separation of Alfrom the solid solution occurs, and Al is precipitated (FIG. 11B). At aheating temperature of 600° C. or higher, the precipitated Al diffusesto the surface (FIG. 11C). It was found that in a heating temperaturerange of 600° C. to 800° C., performing the acid treatment after theheating treatment makes it possible to reduce the Al contained in Si byabout 90%. It is considered that at a heating temperature of 900° C. orhigher, Al having diffused on the surface may be oxidized together withSi, turn into an (Si, Al) oxide, and remain on the surface even afterthe acid treatment (FIG. 11D). The (Si, Al) oxide is considered to beelectrochemically inert, and assumed to have a function of strengtheningthe silicon skeleton by covering the surface of the active material.

Preparation of Power Storage Device

Next, the charge and discharge characteristics of the power storagedevice using the silicon materials of Experimental Examples 8 to 12 as anegative electrode active material were examined. Each of the negativeelectrode active materials (60% by mass) of Examples 8 to 12, 20% bymass of acetylene black having an average particle size of 2 μm as aconductive material, and 20% by mass of polyimide as a binder were mixedtogether, N-methylpyrrolidone (NMP) as a solvent was added thereto, andthe mixture was stirred, thereby preparing a slurry. The slurry was thenapplied to a copper foil having a thickness of 20 μm, then dried, androlled, thereby preparing a negative electrode having a thickness of 50μm. It was assumed that the rolling might reduce the porosity of theporous silicon material to about 50% by volume while allowing the poroussilicon material to maintain the three-dimensional network structure.The prepared negative electrode was punched into circles having adiameter of 16 mm, a porous polyethylene separator was interposedbetween the negative electrodes, metallic lithium was stacked thereon asa counter electrode, and an electrolytic solution was injected into thelaminate, thereby preparing a Tomcell-type small lithium secondarybattery as a test cell. The electrolytic solution used was prepared byadding LiPF₆ at a concentration of 1 mol/L to a mixed solvent obtainedby mixing together fluoroethylene carbonate/ethylene carbonate/dimethylcarbonate/ethyl methyl carbonate (FEC/EC/DMC/EMC) at a volume ratio of1.5:3:4:3. The obtained test cell was repeatedly charged and discharged50 cycles at a current density of 0.1 C and a battery voltage in a rangeof 0.005 V to 1.5 V.

Results of Evaluation on Test Cell Characteristics

Table 4 shows the initial discharge capacity (mAh/g), the dischargecapacity after 50 cycles (mAh/g), and the capacity retention rate (%)obtained from the discharge capacity after 50 cycles with respect to theinitial discharge capacity. As shown in Table 4, by the examination onthe initial discharge capacity, it was revealed that the test cells ofExperimental Examples 8 to 11, in which the heat treatment was performedat 1,000° C. in an inert atmosphere after the pore forming treatment,exhibit a discharge capacity of 2,500 mAh/g or more which is a higherdischarge capacity. Regarding the durability of the battery, inExperimental Example 12, the capacity retention rate for 50 cycles was6% which is an extremely low rate. However, in the test cells using theporous silicon materials of Experimental Examples 8 to 11 as a negativeelectrode active material, the capacity retention rate was 84% to 97%which is an excellent result. Therefore, it was found that making thealloy into a porous material and performing a heat treatment makes itpossible to further increase the capacity and the cycle capacityretention rate. It was assumed that by the heat treatment, Al present asa solid solution in the silicon skeleton may be removed as shown in FIG.12 , and the purer Si skeleton may further improve the capacity. It wasassumed that the formation of the Al oxide on the surface of the siliconskeleton may further strengthen the silicon skeleton, which may lead tofurther improvement of durability, such as volume change by the chargeand discharge cycles.

TABLE 4 Capacity Non- Discharge retention aqueous Initial capacity rateelectrolytic discharge after 50 for 50 solution capacity cycles cycles¹⁾cell mAh/g mAh/g % Example 8 2824 2457 87.1 Example 9 2705 2293 84.8Example 10 2716 2349 86.5 Example 11 3011 2517 83.6 Example 12 2804 1655.9 ¹⁾Measured 50 cycles at 0.1 C in range of 0.005 V to 1.5 V andcalculated from Q₅₀/Q₁ × 100 where Q₁ represents initial capacity(mAh/g) and Q₅₀ represents capacity (mAh/g) in 50th cycle

Note that the present disclosure is not limited to the experimentalexamples described above. It goes without saying that the presentdisclosure can be embodied in various aspects as long as the aspects arewithin the technical scope of the present disclosure.

The present disclosure is applicable to the technical field of secondarybatteries.

What is claimed is:
 1. A power storage device comprising: a positiveelectrode that contains a positive electrode active material; a negativeelectrode that contains the porous silicon material as a negativeelectrode active material; and an ion conducting medium that isinterposed between the positive electrode and the negative electrode andconducts carrier ions; wherein the porous silicon material comprisesskeletal silicon with a three-dimensional network structure having voidsthat are formed of silicon having a lattice constant of 5.435 Å or less,wherein: an average porosity of the porous silicon material is in arange of 50% by volume or more and 95% by volume or less; a proportionof Si contained in the porous silicon material as an element excludingoxygen is 85% by mass or more; a proportion of Al contained in theporous silicon material as a first element is in a range of 15% by massor less; and Al is present on a surface of the porous silicon material.